Ball valves are known which consist of a substantially cylindrical valve body and a rotatable ball-shaped valve element which is drilled and actuated to the closed and opened positions by an operating handle through an operating shaft. In ball valves of known type the valve body is provided, on one side, with a sort of inwardly extending flange, acting as a seat for an annular gasket which bears against the ball-shaped valve element on the other side. On the opposite side, a second annular gasket for the rotatable ball is supported by a rest bushing inserted inside the valve body. The flanged ends of the pipes converging on the valve are tightened to both ends of the valve body by means of lock rings which are screwed to the external surface of the valve body.
This kind of arrangement allows to remove the valve in order to verify the state of wear of the gaskets, without removing either valve duct.
However, in some cases, it may be necessary to disassemble the pipe connected to one of the two sides of the valve, the valve being closed, while in the other pipe a fluid is still under pressure. Because of this necessity, it is compulsory to place the valve in such a way that the gasket rest bushing is always set upstream of the pressure side; otherwise, the bushing, the related gasket and the valve ball itself, could be shot out of the valve body by the pressure on the other side of the valve.
Such type of known valve has two defects: first of all, when assembling it, the flow direction of the fluid should be known: but even so an assembling mistake is always possible; secondly the pipe may be inserted into a complex circuit where, depending on the system regulating conditions, the pressure can be indifferently applied on either side of the valve.